Article pubs.acs.org/JPCC
D2O Interaction with Planar ZnO(0001) Bilayer Supported on Au(111): Structures, Energetics and Influence of Hydroxyls Xingyi Deng,*,†,‡ Dan C. Sorescu,† and Junseok Lee†,‡ †
National Energy Technology Laboratory (NETL), United States Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States ‡ AECOM, P.O. Box 618, South Park, Pennsylvania 15129, United States S Supporting Information *
ABSTRACT: We investigate the interaction between D2O and the planar ZnO(0001) bilayer grown on Au(111) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We show that D2O molecules adsorbed on this planar surface form two ordered overlayers, a (3 × 3) and a (√3 × √3)R30°, not seen before on any of the bulk ZnO single crystal surfaces. The apparent activation energies of desorption (Ed) estimated from TPD peaks are 15.2 and 16.7−17.3 kcal/mol for (3 × 3) and (√3 × √3)R30°, respectively, which agree well with the adsorption energy values calculated from DFT (14.9−15.6 kcal/mol and 16.8−16.9 kcal/mol, respectively). The DFT calculations reveal that the formation of the overlayers takes place at different packing densities and is mediated by extensive hydrogen bonding among the molecules. The hydroxyl groups, which accumulate very slowly on the ZnO(0001) bilayer surface under the standard ultrahigh vacuum (UHV) environment, strongly suppress the formation of the (√3 × √3)R30° overlayer but have less impact on the (3 × 3) overlayer. These findings are explained based on the difference in packing densities of the overlayers such that only the (3 × 3) overlayer with a more open structure can accommodate small amounts of the adsorbed hydroxyl groups.
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film.29 Further examination of the surface chemistry of these novel ultrathin oxide films is of interest for their potential applications in heterogeneous catalysis. Interaction of water with ZnO plays a crucial role in water gas shift reaction and methanol synthesis.30−32 Hence, much of the effort has been directed to understanding the fundamental details of water interaction with ZnO single crystal surfaces. For this purpose, several low Miller index surfaces have been investigated including two polar (ZnO(0001)-Zn and ZnO(0001)̅ -O) and two nonpolar (ZnO(101̅0) and ZnO(112̅0)) surfaces. Not surprisingly, water interacts differently with these surfaces as reported in literature.33−35 Briefly, dissociative adsorption occurs on polar ZnO surfaces,36−38 whereas a “half-dissociated” water structure leading to a stable hydrogen-bonded network was proposed to exist on the nonpolar ZnO surfaces.39,40 In addition, calculations also revealed the existence of a molecular adsorption structure on ZnO(101̅0)41 and a fully dissociated structure on ZnO(112̅0).42 Notably, the clean ZnO(0001̅)-O with a (1 × 3) reconstruction is very reactive and converts to a fully hydroxylated H(1 × 1) ZnO(0001)̅ surface upon reaction with
INTRODUCTION Currently, there is considerable interest in the growth and characterization of oxide nanostructures and thin films. Investigating the surface structures and properties of these oxide nanostructures and thin films is a crucial step toward advancing their applications in heterogeneous catalysis.1,2 A variety of oxide thin films, including MgO,3−6 Al2O3,7−11 ZrO2,12 and Fe-oxides,13−21 have been reported in literature. Interestingly, oxide materials in the ultrathin regime, consisting of single or a few atomic layers could lead to emergence of new types of materials, with properties different from their bulk counterpart. Following theoretical predictions,22,23 recent experiments have demonstrated the possibility to grow planar, graphite-like ultrathin ZnO(0001) films on Ag(111),24,25 Pt(111), 26 Pd(111),27 and Au(111)28 in which the novel structure of the oxide film is driven by a mechanism to depolarize the nonvanishing dipole moment at the surface resulting from the alternating Zn and O layers in the bulk wurtzite structure. More recently, we have explored single- and few-layer ZnO(0001) nanostructures grown on Au(111) with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations and found that the lattice constants and band gaps of the ultrathin ZnO(0001) are tunable by controlling the number of layers in the oxide © XXXX American Chemical Society
Received: January 26, 2016 Revised: March 23, 2016
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DOI: 10.1021/acs.jpcc.6b00862 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. STM images of (a) 0.3 MLE (100 × 100 nm2, V = 1.5 V, I = 5 pA), (b) 0.8 MLE (100 × 100 nm2, V = 1.5 V, I = 5 pA), and (c) 1.8 MLE (300 × 300 nm2, V = 1.5 V, I = 5 pA) ZnO bilayer on Au(111). The ZnO bilayer was prepared using the reactive deposition method described in the text. STM images were taken at room temperature.
bilayers on Au(111) was achieved by reactive deposition of Zn in the presence of NO2 (P = 3 × 10−8 mbar) at T = 420 K. Zn was evaporated using an electron-beam assisted evaporator (Omicron EFM3T) from a Zn rod (Goodfellow, 2.0 mm diameter, 99.99%). NO2 (Research grade) was used as received. The coverage of ZnO on Au(111) is noted as a monolayer equivalent (MLE) and was determined from the XPS results as described previously.28 XP spectra were collected using a MgKα X-ray source (1253.6 eV, 300 W) and a hemispherical analyzer with a pass energy of 20 eV. The binding energy was calibrated with the Au 4f7/2 peak at 83.8 eV for each spectrum. The XPS data were analyzed by using Shirley background subtraction and curve fitting with mixed Gaussian−Lorentzian functions to determine the peak areas. LEED experiments were carried out at T = 100 K with a beam energy of ∼42 eV. STM measurements were performed at room temperature in the constant current mode using etched W tips. All STM images were plane-corrected using scanning probe imaging processor (SPIP, Imagemet) software. The TPD data were collected by a computer-controlled MKS Microvision-IP mass spectrometer. The temperature was controlled by a Eurotherm PID unit with a heating rate of 2 K/s. D2O was introduced into the chamber through a UHV leak valve with a stainless steel tube whose end is located 10 cm away from the sample surface (an enhancement factor of 4 was estimated for this direct dosing geometry as compared to the background exposing). The D2O exposures, noted as Langmuir (1 L = 1.33 × 10−6 mbar·s), were controlled by varying the time with a pressure rise of 5 × 10−10 mbar at T = 100 K. D2O (99.994 atom % D, Aldrich) was purified with cycles of freeze−pump−thaw treatment. The interaction of H2O overlayers with the ZnO bilayer has been also investigated theoretically using Vienna ab initio simulation package (VASP).44,45 The gradient-corrected PBE functional46 was used for the treatment of the exchange and correlation together with the PAW method of Blöchl47 for description of electron−ion interaction. The DFT calculations were corrected for on-site Coulomb interactions with the GGA +U procedure,48 with an effective Ueff = U − J of 8.5 eV for Zn. This value was shown previously49 to improve the predictions of the electronic properties for bulk and ultrathin films of ZnO. The wave functions were expanded in a plane-wave basis set with a cutoff energy of 400 eV. The standard GGA calculations were
residual water in a standard ultrahigh vacuum (UHV) environment.37,43 In this work, we investigate the structures and energetics of D2O overlayers on the ZnO(0001) bilayer (hereafter denoted in a simplified way as a ZnO bilayer) in an attempt to explore the unique surface chemistry that could arise from the planar characteristic of the ultrathin film. D2O was used because of the much lower background of the m/z = 20 signal in the temperature-programmed desorption (TPD) experiments compared to that of H2O (m/z = 18). When adsorbed on the ZnO bilayer surface, D2O molecules form two hexagonal overlayer structures, that is, a (3 × 3) and a (√3 × √3)R30°, as determined from the TPD and low energy electron diffraction (LEED) experiments. We find no direct evidence of enhanced adsorption or dissociation of D2O at the interfacial edges of the ZnO bilayer (referring to the boundaries of the ZnO nanostructures). The formation of both D2O overlayers is mediated by hydrogen bonding, and their structures and energetics were evaluated based on DFT calculations. We also find that the dissociation of D2O is not a facile process on the surface of the ZnO bilayer although the accumulation of the hydroxyls does occur with a very slow rate under a standard UHV condition (P < 3 × 10−10 mbar). The presence of the hydroxyls on the ZnO bilayer surface suppresses the formation of the (3 × 3) and (√3 × √3)R30° overlayers with the latter being mostly affected. These results differ remarkably from those obtained in previous studies on the wurtzite ZnO single crystal surfaces35 and hint to the possibility of exploiting materials in the ultrathin regime with new physical and chemical properties.
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EXPERIMENTAL AND THEORETICAL METHODS
The experiments were performed in two separate commercial UHV chambers (Omicron Nanotechnology GmbH). Both chambers are equipped with LEED optics, X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS). One chamber is also equipped with a variable-temperature STM with a base pressure of 2 × 10−10 mbar, while the other one (base pressure: 3 × 10−10 mbar) has a differentially pumped mass spectrometer (MKS Microvision-IP) for TPD experiments. In both chambers, the clean Au(111) surface (10 × 10 × 1 mm3, 99.999% purity, Princeton Scientific Corp.) was prepared by cycles of Ar+ sputtering (1.5 keV) at 300 K, followed by annealing at T = 700 K for 10 min. The growth of the ZnO B
DOI: 10.1021/acs.jpcc.6b00862 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. (a) TPD spectra of D2O (m/z = 20) from 1.3 MLE ZnO bilayer on Au(111) following the D2O exposure of 0.1−0.2 L (1 L = 1.33 × 10−6 mbar· s) at T = 100 K. The desorption peaks at T = 245 (β) and 270 K (γ) are unique features that are not present on the ZnO single crystal surfaces; (b) a series of TPD spectra from this surface at much higher D2O exposures (up to 1.5 L), in which the D2O multilayer desorption peak is observed at T = 163 K. The ramping rate was 2 K/s; (c) O 1s XP spectra of 1.3 MLE ZnO bilayer on Au(111) following adsorption of 1 L D2O at T = 100 K and subsequent heating (2 K/s) to temperatures indicated in the figure. There is no other O 1s peak in the XP spectrum (excepting the peak at BE = 529.4 eV corresponding to the lattice oxygen from ZnO) after heating to 600 K.
continues to grow, leading eventually to formation of much larger structures covering >80% of the Au(111) surface at 1.8 MLE (Figure 1c). However, portions of the ZnO bilayer surface can also become covered with additional ZnO layers, as shown in Figure 1c. Multiple D2O desorption peaks were detected in the TPD measurements following the adsorption of D2O at T = 100 K on the ZnO bilayer grown on Au(111) (1.3 MLE). At a D2O exposure of 0.1 L, we observed two overlapped desorption peaks (m/z = 20) located at T = 245 (labeled as β) and 270 K (labeled as γ), respectively (Figure 2a). These two peaks saturate quickly, and a sharp D2O desorption peak develops at T = 205 K (labeled as α) at the D2O exposure of 0.2 L, as also shown in Figure 2a. The α peak continues to grow and slightly shifts to T = 207 K upon increasing D2O exposure up to about 0.5 L, and an additional desorption peak emerges near T = 160 K upon further increase of the D2O exposure to 1.0 L (Figure 2b). This latest desorption peak continues to grow with the D2O exposure and is unsaturable, and thus attributed to the sublimation of the D2O multilayers. The desorption peak α at T = 205−207 K is assigned to the chemisorbed D2O on the ZnO bilayer. For comparison, desorption of the chemisorbed H2O from the single crystal ZnO(0001)̅ -O surface occurs at T = 207 K.52 No other chemical species, including D2 (m/z = 4) and O2 (m/z = 32), were detected in TPD up to 600 K (data not shown). Note that the TPD data have been acquired only up to T = 600 K to avoid structural changes in the ZnO bilayer occurring at T > 600 K. As will be discussed below no detectable species are left on the surface after heating to 600 K. The O 1s XP spectra of the ZnO bilayer (1.3 MLE) following adsorption of D2O at T = 100 K and subsequent heating to three different temperatures are displayed in Figure 2c. The XP peak at binding energy (BE) = 529.4 eV is from the lattice O of the ZnO bilayer and thus it is seen in all spectra. A broad XP peak centered at BE = 532.4 eV which appears after the D2O exposure at 100 K is assigned to the O 1s of the intact, adsorbed D2O. This XP peak becomes smaller as a result of desorbing the D2O multilayers after heating to T = 165 K whereas its BE remains unchanged at 532.4 eV. After heating to T = 215 K, the XP peak is shifted to a slightly lower BE of 532.0 eV and continues to lose intensity due to desorption of the chemisorbed D2O (TPD peak α at T = 207 K). There is no detectable amount of other O species, that is, the hydroxyls, left on the surface (except for lattice O) after heating to T = 600 K, indicating that the dissociation of D2O is not facile on the planar ZnO bilayer.
also corrected to include long-range dispersion interactions. For this purpose, we tested both the Grimme’s D350 and the Tkatchenko and Scheffler methods51 to describe the van der Waals interactions. Hereafter we denote these methods as PBED3 and PBE-TS, respectively. The coverage dependence of H2O overlayers was studied by using slab models of different sizes, that is, (7 × 7), (3 × 3), or (√3 × √3)R30°, respectively. In all instances, the in-plane lattice constant of the supercell was taken from the results we obtained previously29 in the case of (9 × 9) ZnO bilayers/(16 × 16)Au where we showed that the optimized in-plane lattice parameters of ZnO were very close to the experimental STM data.29 In the current work, for the lowest coverage (corresponding to a single water molecule adsorbed in the (7 × 7) supercell), we have tested the dependence of the adsorption energy on slab models with and without Au support. In the former case, the Au support contained four layers among which the top two were relaxed, while the bottom two were kept frozen at bulk optimized configuration. As will be shown in this work, the Au substrate has only a small influence on the calculated adsorption energies relative to the results obtained using a simplified model with only the ZnO bilayer. For this reason, in calculations performed at higher water coverages, the surface model contained only the ZnO bilayer without the Au substrate. The adsorption energies of H2O were calculated using Eads = (nEH2O + Eslab − E(H2O+slab))/n, where EH2O is the energy of the H2O molecule at its optimized gas-phase geometry, n represents the number of adsorbate molecules in the simulation cell, Eslab is the total energy of the slab, and E(H2O+slab) is the total energy of the adsorbate/slab system. In this sign convention, positive adsorption energies correspond to stable configurations. The energy of the isolated H2O molecule was calculated using a cubic cell with sides of 12 Å.
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RESULTS AND DISCUSSION The ZnO bilayer is the dominant structure grown on Au(111) via reactive deposition of Zn in NO2 at 420 K, as shown in Figure 1. The ZnO bilayer on Au(111) adopts a planar, graphite-like structure, showing an apparent height of 3.6 Å and a hexagonal Moiré pattern with a periodicity of ∼23 Å in STM, as described in our previous work.29 At low ZnO coverage of 0.3 MLE, most of the bilayers appear as features with triangular shapes and with sizes in the range of 10−15 nm (Figure 1a). The average edge length of these bilayer ZnO structures grows to ∼40 nm as the coverage increases to 0.8 MLE (Figure 1b). The ZnO bilayer C
DOI: 10.1021/acs.jpcc.6b00862 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. TPD spectra of D2O (m/z = 20) from (a) 0.3 MLE and (b) 1.9 MLE ZnO bilayer on Au(111) following adsorption of D2O at T = 100 K. The heating rate used in these experiments was 2 K/s; (c) areas of saturated D2O desorption peaks β, γ, and β + γ, and (d) peak γ to peak β area ratio (γ/β) as a function of ZnO coverages. The peak areas were estimated by fitting the selected TPD traces as described in Supporting Information.
Figure 4. LEED patterns of (a) a (3 × 3) D2O superstructure on the ZnO bilayer (prepared by an exposure to 0.3 L D2O on 1.1 MLE ZnO at T = 100 K and subsequent heating to T = 200 K); (b) a (√3 × √3)R30° D2O superstructure on the ZnO bilayer (prepared by an exposure to 0.05 L D2O on 1.1 MLE ZnO at T = 100 K and subsequent heating to T = 200 K); the ideal LEED patterns of (c) (3 × 3) and (d) (√3 × √3)R30° superstructures. The lattice directions of the ZnO(0001) bilayer and D2O superstructures are marked with red and white arrows, respectively. All LEED patterns were obtained at T = 100 K with a beam energy of 42 eV. The contrast in the LEED images was adjusted to enhance the visibility.
is absent in the TPD trace shown in Figure 3a. As the ZnO coverage increases to 1.9 MLE, both the β and γ peaks gain significant intensity with the peak γ slightly shifting to T = 275 K, as shown in Figure 3b. In addition, a non-negligible D2O desorption peak evolves at T = 400 K (indicated with an arrow in
The relative amounts of D2O that saturate the desorption peaks β and γ depend on the ZnO coverage. On the surface of 0.3 MLE ZnO bilayer, an exposure of 0.04 L D2O is enough to saturate the peak β and to start developing the chemisorbed D2O desorption peak α at T = 205 K (Figure 3a). Note that the peak γ D
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Figure 5. Top views of the optimized structures of a water monomer in the (7 × 7) ZnO supercell for (a) a slab containing a combined ZnO(7 × 7) bilayer/Au(111)(8 × 8) model and (b) a slab containing only the ZnO(7 × 7) bilayer. Representative adsorption configurations of water molecules on the planar ZnO bilayer with the (3 × 3) (c−e) and (√3 × √3)R30° (f, g) superstructures. For the (3 × 3) overlayer the number of water molecules per unit cells is 5 (c), 6 (d), and 6 (e), respectively, while for the (√3 × √3)R30° overlayer the number of molecules in the unit cell is 3 (f) and 4 (g), respectively. For each configuration, the adsorption energies calculated using the PBE-D3 [PBE-TS] method are indicated in kcal/mol. For improved visualization, the Zn and O atoms of ZnO are colored in green and red in (a) and (b) and as light blue and yellow in (c−g). The O and H atoms of H2O are colored in red and white, respectively, in all panels. The increasingly darker color scheme used for Au atoms in panel (a) denotes Au atoms located at increasingly deeper layers.
Au(111)-(8 × 8) along the surface lattice directions. After an exposure of 0.3 L D2O at T = 100 K, the diffraction spots of the ZnO(0001)-(1 × 1) lattice become less visible and their surrounding hexagonal spots disappear in LEED (Figure S2b). Upon heating the surface to T = 200 K to desorb the chemisorbed D2O (the peak α in TPD), the intensity of the ZnO(0001)-(1 × 1) diffraction spots is partially recovered, and more importantly, new diffraction spots with a hexagonal symmetry appear in the LEED pattern (Figure 4a). The lattice direction of these new diffraction spots (represented by white arrows) is aligned along that of the ZnO(0001) diffractions (represented by red arrows) with a 1/3 of the unit cell size, suggesting a (3 × 3) D2O overlayer structure on the ZnO bilayer. By decreasing the initial D2O exposure to 0.05 L at T = 100 K, followed by heating to T = 200 K (to minimize the population of D2O in β), a D2O overlayer with a (√3 × √3)R30° superstructure is observed in LEED (Figure 4b). For comparison, the ideal (3 × 3) and (√3 × √3)R30° LEED patterns (simulated by LEEDpat version 4.1 by K. E. Hermann and M. A. Van Hove) are shown in Figure 4c and d, respectively. Since the diffraction pattern of (√3 × √3)R30° is a subset of that of (3 × 3), the D2O overlayers with both superstructures could coexist in Figure 4a. Furthermore, the LEED patterns are restored to the original coincidence structure of ZnO(0001)-(7 × 7)/Au(111)-(8 × 8) after heating the surface to T = 600 K to desorb the β and γ peaks (Figure S2c). Based on the TPD and LEED data, we propose that the β and γ desorption peaks are associated with formation of a (3 × 3) and a (√3 × √3)R30° D2O overlayers on the ZnO(0001) bilayer, respectively. The O 1s BE shift observed in Figure 2c (532.4 eV → 532.0 eV) is likely associated with the transition from the disordered chemisorbed water to the ordered water overlayers (i.e., (3 × 3) and (√3 × √3)R30°). The apparent activation energies of desorption are 15.2 (Tβ,peak = 245 K) and 16.7−17.3
Figure 3b) on this surface. Such a desorption peak was not observed in the TPD traces at lower ZnO coverages. Since additional ZnO layers are formed on the ZnO bilayer at higher ZnO coverages (Figure 1c), we tentatively attribute the peak at T = 400 K to desorption of D2O from surfaces of ZnO films with additional (n > 2) layers or steps formed between these layers. The areas of saturated β and γ peaks as a function of the ZnO coverages are summarized in Figure 3c (see Supporting Information for examples of the TPD trace fits). Both the β and γ peak areas, as well as the total peak areas (β + γ), increase monotonically with the increase of the ZnO coverage up to 1.9 MLE. Notably, at the ZnO coverage of 1.9 MLE, most of the Au(111) surface is covered by ZnO such that the ZnO/Au interface has a diminishing role (Figure 1c). This suggests that both the β and the γ peaks arise from desorption of D2O from the basal planes of the planar ZnO bilayer and not from the interfacial edges of the ZnO bilayer with Au. We also find that the γ peak grows more rapidly than the β peak with the ZnO coverage. Specifically, the saturated peak area ratio γ/β increases from 0 to 0.42 as the coverage of the ZnO bilayer increases from 0.3 MLE (average size 10−15 nm) to 1.9 MLE (larger islands), as shown in Figure 3d. This suggests that formation of the D2O structure(s) associated with the desorption peak γ strongly depends on the average size of the ZnO bilayer and becomes more facile on the larger-sized surfaces. We will discuss this aspect in more details later. Long-range ordered D2O overlayer structures are formed on the planar bilayer ZnO as revealed by the LEED data shown in Figure 4. The LEED pattern of the clean ZnO bilayer on Au(111) (1.1 MLE) consists of hexagonal diffraction spots of Au(111) aligned with those of ZnO(0001)-(1 × 1), which are also surrounded by an additional set of hexagonal spots associated with the Moiré pattern as seen in STM (Figure S2a). This is consistent with a coincidence structure of ZnO(0001)-(7 × 7)/ E
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Figure 6. (a) O 1s XP spectra of freshly prepared 0.9 MLE ZnO bilayer on Au(111), and after 4 and 14 days in the UHV environment at room temperature (base pressure: 3 × 10−10 mbar), indicating slow accumulations of the hydroxyls on the ZnO surface; (b) TPD of D2O (m/z = 20) from 0.9 MLE ZnO bilayer precovered with 0, 4, and 7.5% of the hydroxyls (as determined from (a)) on the surface; (c) areas of D2O desorption peaks β and γ estimated from (b) as a function of the hydroxyl surface atomic concentrations; (d) TPD of D2O (m/z = 20) from 0.9 MLE ZnO bilayer precovered with 7.5% hydroxyls (heated up to T = 700 K to remove the hydroxyls by desorbing them as water at T = 650 K) and the subsequent D2O TPD experiment from the hydroxyl-free surface. These results demonstrate that the hydroxyls strongly suppress the desorption peak γ and have less impact on the desorption peak β. In all TPD experiments, 0.1 L (1 L = 1.33 × 10−6 mbar·s) of D2O exposure was used at T = 100 K. The heating rate was 2 K/s.
away from this site and binds at the Zn site in a configuration similar to that indicated in Figure 5a,b. In the case of the (3 × 3) water overlayer, several configurations are possible depending on the water coverage. We illustrate three such cases in Figure 5c−e. The water overlayer structure in Figure 5c with five molecules in the (3 × 3) unit is a honeycomb structure. The calculated adsorption energies are 14.9 and 14.7 kcal/mol using PBE-D3 and PBETS methods, respectively. Each hexagon contains 12 water molecules: six adsorbed laying down on the surface with oxygen binding to the Zn sites and forming the hexagon corners, and six residing in the middle of the hexagonal sides and bonded via hydrogen bonds to neighbor water molecules. The increase in the number of water molecules to six per (3 × 3) unit cell can lead to different packing patterns. Two such models with the most stable configurations are depicted in Figure 5d,e. In Figure 5d, the structure contains interconnected water pentagons via water monomers, while in Figure 5e the corresponding structure contains an ensemble of complex rings containing tetramers interconnected by water monomers and dimers. Further increase in water coverage leads to formation of new structures with the (√3 × √3)R30° symmetry. Such overlayers are more complex, and in fact several structures are possible. We illustrate two of the most stable cases identified in Figure 5f,g. These structures have three and respectively four molecules per (√3 × √3)R30° unit cell (equivalent to 9 and, respectively, 12 molecules per (3 × 3) unit cell). Their adsorption energies of 16.8 and 16.9 kcal/mol, respectively, are higher than those obtained in the case of the (3 × 3) overlayer. Both structures contain water molecules interconnected by multiple hydrogen bonds, leading to an overall more compact packing than that of the (3 × 3) overlayers shown in Figure 5c−e. The results from the DFT calculations presented in Figure 5 show that the adsorption energies of water overlayers on the planar ZnO bilayer can be ordered as follows: isolated molecules
(Tγ,peak = 270−279 K) kcal/mol assuming a pre-exponential factor of 1 × 1013 s−1 and first-order kinetics53 with a heating rate of 2 K/s. Accordingly, the apparent activation energy for chemisorbed D2O desorption (Tα,peak = 205 K) from the ZnO bilayer is 12.6 kcal/mol. Note that the activation energy for chemisorbed water desorption (Tpeak = 207 K, heating rate ∼5 K/ s) from a bulk single crystal ZnO(0001̅)-O surface was estimated to be 14.4 kcal/mol assuming a pre-exponential factor of 1 × 1015 s−1.52 The structures and energetics of the water overlayers on the planar ZnO bilayer were further studied by DFT calculations. Figure 5a,b compares the configuration of a water monomer for the case of two slab models containing a combined ZnO(7 × 7) bilayer/Au(111)(8 × 8) and respectively, only a ZnO(7 × 7) bilayer. In the case of the surface model with the Au support included, we find no major dependence of the adsorption energy of the water monomer on the ZnO binding site relative to the Au substrate, that is, fcc, hcp, or top regions, and thus, a single generic adsorption configuration is indicated in Figure 5a, where an O atom of the water monomer is located above the Zn atom. By comparing the results for the models with (Figure 5a) or without (Figure 5b) the Au substrate, we see only small differences between the two sets of results for either the binding configurations or their adsorption energies. Furthermore, the similarity of the results obtained is confirmed regardless of the theoretical method used, that is, PBE-D3 or PBE-TS. These finding suggest that the effect of Au substrate upon the energetics of water adsorption on the ZnO bilayer is minimal. For this reason, the Au substrate was eliminated in the following analysis. We note that configurations shown in Figure 5a,b correspond to the water monomer binding at the Zn site on the planar ZnO bilayer surface. The water monomer binding to the oxygen site is not favorable. Regardless of the initial configuration of water monomer at the O site, upon optimization, the molecule moves F
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Figure 7. (a) Selected O 1s and (b) Zn 2p XP spectra of 0.6, 1.3, and 1.9 MLE ZnO bilayer grown on Au(111) using the reactive deposition method described in the text; (c) O-to-Zn atomic ratio as a function of the ZnO coverage. The ratio continuously decreases and eventually approaches the stoichiometric ZnO with the increase in the ZnO coverage. The O-to-Zn atomic ratio has been obtained using the method described previously.28
< (3 × 3) < (√3 × √3)R30°, consistent with our TPD and LEED experiments. This order of stability is confirmed by both PBE-D3 and PBE-TS computational methods. Furthermore, the calculated adsorption energies of the (3 × 3) overlayers with values in the range 14.9−15.6 kcal/mol match very well the corresponding experimental value of 15.2 kcal/mol estimated from the TPD peak analysis. Similarly, in the case of the (√3 × √3)R30° overlayers the calculated adsorption energies of 16.8− 16.9 kcal/mol are very close to the experimental values of 16.7− 17.3 kcal/mol. The hydroxyl groups play important roles in heterogeneous catalysis, for example, as crucial intermediates in reactions54,55 or to enhance the catalyst reactivity.56,57 For this reason, we investigated the influence of the hydroxyl groups on the ZnO bilayer. While not detected on the surfaces of the freshly prepared ZnO bilayer or following D2O experiments, the hydroxyls groups are observed if the freshly prepared ZnO bilayer is intentionally left in the UHV chamber for extended periods of time at room temperature, as demonstrated in Figure 6a. Specifically, the XP spectrum of the freshly prepared ZnO bilayer in the O 1s region consists of a single peak centered at BE = 529.4 eV due to the lattice oxygen. After 4 days in the UHV chamber, a small peak centered at BE = 532.2 eV (∼4% compared to the lattice oxygen) appears in the O 1s region and is attributed to the hydroxyls (ODa and OHa). The area of the hydroxyl peak at BE = 532.2 eV slowly increases to ∼7.5% of the area of the peak from the lattice oxygen after 14 days. We speculate that the hydroxyls are produced via dissociation of background water at the sparse defect sites, slowly diffusing to populate the surface of the ZnO bilayer. We find that the presence of the hydroxyls strongly influences the β and γ peaks in the TPD experiments. Figure 6b displays the TPD traces from the freshly prepared ZnO bilayer surface (0.9 MLE, nearly hydroxyl free) following exposure to 0.1 L D2O, and the same surface precovered with 4 and 7.5% hydroxyls. Both the β and γ peaks are clearly seen in TPD spectra from the freshly prepared surface with no detectable hydroxyls. The appearance of the chemisorbed D2O peak α at T = 205 K in TPD indicates that β and γ peaks have been saturated at this exposure. Notably, when the ZnO surface is precovered with ∼4% hydroxyls, the peak β becomes slightly smaller but the peak γ completely disappears in TPD. Further increase of the precovered hydroxyls to 7.5% makes the peak β continue to drop in intensity while the peak γ is still absent in TPD. To semiquantify these results, the saturated peak areas of β and γ as a function of the atomic concentrations of the hydroxyls are summarized in Figure 6c. More importantly, both the β and the γ peaks are partially
recovered in TPD following the hydroxyl removal via thermal desorption at T = 650 K (recombinative desorption as water) and the subsequent D2O exposure, as shown in Figure 6d. Since heating to T = 700 K causes partial structural changes in the ZnO bilayer leading to extra ZnO layers (data not shown), a small but visible D2O desorption peak at T = 400 K is also shown in the TPD trace, likely associated with desorption from the surface of additional ZnO layers (beyond bilayer) or from the steps formed between these layers. The influence of the hydroxyls on the evolutions of the β and γ peaks in TPD could be explained by considering the structural characteristics of the molecular configurations responsible for these desorption peaks. As discussed above, the β and γ desorption peaks are associated with the (3 × 3) and (√3 × √3) R30° water overlayers on the planar ZnO bilayer, respectively. As seen from DFT calculations (Figure 5c−e), the (3 × 3) overlayers have plenty of uncovered surface sites within the unit cell, whereas the (√3 × √3)R30° overlayers (Figure 5f,g) are much more compact and very few surface sites are unoccupied. Given the more compact nature of the (√3 × √3)R30° water overlayer, a small amount of the hydroxyls may be able to suppress the formation of this overlayer structure on the ZnO bilayer. On the other hand, the (3 × 3) overlayers could accommodate the hydroxyls inside their open structures, making the overlayer structure associated with the peak β less vulnerable to adsorption of the hydroxyls. The influence of the hydroxyls could also explain the change of the peak area ratio of γ/β as a function of the ZnO coverage, as shown above in Figure 3c,d. Basically, the average size of the ZnO bilayer structures grows with the increase of the ZnO coverage. It is likely that a small amount of the hydroxyls below the detection limit of XPS might be present on the freshly prepared ZnO bilayer at all coverages. Such small amounts of the hydroxyls may not seem significant on the larger or extended ZnO bilayer surfaces, but would be enough to disturb the continuity of the hydroxyl free areas (required for the formation of the (√3 × √3)R30° water overlayers) on the smaller ZnO bilayer surfaces (for example, 0.3 MLE ZnO bilayer with the size range of 10−15 nm). For this reason, the peak γ was not detected in TPD on the 0.3 MLE ZnO bilayer, but was observed for larger ZnO coverages and grew rapidly with the increase of the ZnO coverage/size. The results and the assignments presented in this work differ from those obtained in the previous D2O TPD study on ZnO single crystals surfaces,33 in which desorption of D2O from the O and Zn sites was found to occur at 190 and 340 K, respectively. These differences could be attributed to the planar structure of the ZnO bilayer in this study versus the wurtzite ZnO structure in G
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The Journal of Physical Chemistry C previous work. We also suggest that the planar nature of the ZnO bilayer is likely the key leading to the formation of the (3 × 3) and (√3 × √3)R30° water overlayers. As such, these two water overlayer structures have not been observed previously on the single crystal surfaces of ZnO, including the ZnO(0001)-Zn, ZnO(0001̅)-O, and ZnO(101̅0) surfaces. It should be noted that the detailed molecular structure of the (√3 × √3)R30° overlayer on the planar ZnO bilayer is also quite different from that commonly formed on hexagonal metal single crystal surfaces, such as Ru(0001).58,59 This could be attributed to the larger unit cell of the planar ZnO(0001) (3.3 Å) as compared to that of the typical metal surfaces (2.7−2.9 Å). Finally, an aspect briefly investigated in this work is related to the question if the interfacial edges of the ZnO bilayer are active for water dissociation. It is known that the interfacial edges of other ultrathin oxide films grown on metallic support have enhanced reactivity due to the presence of undercoordinated sites.60,61 In the present work, we do not see any evidence for the enhanced adsorption or dissociation of water at the interfacial edges of the ZnO bilayer as all TPD peaks revealed arise from the basal planes of the ZnO bilayer. As shown in Figure 7a,b, where a series of O 1s and Zn 2p XP spectra of the freshly prepared ZnO bilayer with different coverages are displayed, the XP peaks in both regions increase with coverage, but their area ratios change slightly. Specifically, the O-to-Zn atomic ratio is ∼1.08:1 at 0.3 MLE of ZnO and continues to drop as the ZnO coverage increases (Figure 7c). It eventually approaches the stoichiometric value of 1:1 at 1.9 MLE of ZnO. This observation suggests that the interfacial edges of the ZnO bilayer prepared in this work are likely terminated by excess O during the reactive deposition in NO2, thus, resulting in passivation of these edge sites. Additional work is required to fully clarify how such O-terminated edges influence the water adsorption.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES Contract DE-FE0004000. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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CONCLUSIONS D2O forms two ordered overlayers, a (3 × 3) and a (√3 × √3) R30°, on the planar ZnO bilayer which desorb at T = 245 and 270−279 K, as seen from TPD measurements, while the chemisorbed D2O desorbs at T ∼ 205 K. The apparent desorption activation energies estimated from TPD are supported by DFT calculations which also reveal the detailed molecular structures of the (3 × 3) and (√3 × √3)R30° overlayers on the planar ZnO bilayer. The hydroxyl groups, while accumulating on the ZnO surface very slowly, strongly hinder the formation of the D2O overlayers, particularly in the case of (√3 × √3)R30° structure. This finding is correlated with the different packing densities of the two ordered D2O overlayers and the availability of the unoccupied surface sites such that only the (3 × 3) overlayer can accommodate moderate amounts of the hydroxyl groups adsorbed on the ZnO bilayer surface. Furthermore, there is no experimental evidence supporting an enhanced adsorption and facile dissociation of D2O at the interfacial edges of the ZnO bilayer, possibly due to the Oterminated edge sites resulted during the growth of the ZnO bilayer.
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Examples of fitting the TPD traces (Figure S1), and LEED patterns of the clean ZnO bilayer on Au(111) and following adsorption of D2O (Figure S2; PDF).
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00862. H
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